Refrigeration apparatus control

ABSTRACT

A vapor compression refrigerant system is cycled in response to the refrigeration load. The system has a high-pressure-responsive safety protector for terminating operation of the system in response to an abnormally high-refrigerant pressure and a variable-speed condenser fan motor for controlling said refrigerant pressure in response to a variable resistance, said variable resistance being directly responsive to refrigerant pressure whereby said fan operates to maintain the refrigerant pressure below said abnormally high-refrigerant pressure immediately following startup of said compressor and above a predetermined minimum pressure during the run cycle of said system. The pressure-resistive transducer is a piezoresistive semiconductor.

United States Patent [72] Inventor Robert G. Miner La Crosse, Wis. [21] Appl. No. 691,439 [22] Filed Dec. 18, 1967 [45] Patented Jan. 11, 1972 [73] Assignee The Trane Company La Crosse, Wis.

[54} REFRIGERATION APPARATUS CONTROL 10 Claims, 3 Drawing Figs. [52] U.S. Cl. 62/181, 62/184, 62/180, 62/182, 62/183, 318/227, 318/345, 338/4, 338/36 [51] Int. Cl F25d 17/00 [50] Field of Search 62/180, 181, 182, 183, 184; 318/227, 345; 338/4, 36 [56] References Cited UNITED STATES PATENTS 2,939,317 6/1960 Mason 338/4 X 3,160,844 12/1964 McLellan. 338/4 3,270,562 9/1966 Ehrenreich... 338/4 3,329,023 7/1967 338/4 Primary Examiner-William J. Wye Attorneys-Arthur 0. Andersen and Carl M. Lewis ABSTRACT: A vapor compression refrigerant system is cycled in response to the refrigeration load. The system has a high-pressure-responsive safety protector for terminating operation of the system in response to an abnormally highrefrigerant pressure and a variable-speed condenser fan motor for controlling said refrigerant pressure in response to a variable resistance, said variable resistance being directly responsive to refrigerant pressure whereby said fan operates to maintain the refrigerant pressure below said abnormally highrefrigerant pressure immediately following startup of said compressor and above a predetermined minimum pressure during the run cycle of said system. The pressure-resistive transducer is a piezoresistive semiconductor.

PATENTED JAN? 1 m2 alsaalsvs FIG. 2 3 400 400 PSI PSI 300 PSI 300 PSI PSI 30 MIN. 60 MIN. QOMIN. PSI 30 MIN. 60MIN. 9OMIN.

ATTORNEY REFRIGERATION APPARATUS CONTROL BACKGROUND OF THE INVENTION in the art of compression cycle refrigeration systems having air-cooled condensers, it is known to control the condenser fan speed for purposes of maintaining uniform pressure at the refrigerant throttling means despite variations in cooling air temperature and/or load on the condenser. Such condenser fan speed controls have taken several different forms. One scheme of control involves the use of a two-speed fan motor which may be cycled from one discrete speed to another discrete speed as required. Such a system usually requires a switching mechanism for changing the number of active poles in the motor. While this scheme has found widespread use, it will be appreciated that it does not maintain the condenser pressure accurately because the control is by stages rather than by modulation. Further, because of the rather complicated switching requirements, this control is costly and has found little use in residential refrigeration applications.

Modulated condenser fan speed controls have also been known for a number of years. In these systems the condenser fan speed is varied in response to a temperature sensor placed on the condenser in an area of saturated refrigerant where temperature of the refrigerant has been assumed to be directly related to the pressure of the refrigerant. The variation in resistance of the temperature sensor is then used to control a solid-state switch in series with the condenser fan motor thereby controlling the fan speed and thus the temperature or pressure of the condenser. This type of system is satisfactory when operated continuously. However, when operated intermittently, as for example by cycling the compressor in response to the refrigeration load, I have found that the system apparently experiences an excessive transient pressure which may prematurely actuate the high-pressure cutout when outdoor ambient temperatures are relatively high. A possible solution to the problem, of course, would be to place a delay on the high-pressure cutout during startup. However, such a scheme reduces the protection factor of this safety control.

The reason for this excessive transient pressure was not at first fully understood. Although the condenser fan motor current was controlled by an instantaneously acting solid state switch, and the temperature sensor was located on that portion of the condenser known to contain saturated refrigerant, it appeared that the condenser fan was slow in starting. This slow starting may have been because the current passing the solid-state switch would rise gradually and not reach sufficient motor starting level for a period of time. Also the sharp rise in pressure may have been because the cooling effect of the fan on the refrigerant would not be instantaneous due to the thermal lag in emitting heat from the condenser. Possibly a motor could be developed to start at lower currents but little could be done to reduce the thermal lag in heat emission from the condenser.

SUMMARY OF THE INVENTION A solution to the above-mentioned problem was finally found by the use of a pressure-resistive transducer which is capable of sensing the refrigerant pressure directly rather than indirectly. By the use of this device I have avoided high transient starting pressures in the refrigeration system and it is no longer necessary to render the pressure cutout ineffective during starting. The fact that the high starting pressures are eliminated may also be used to advantage in the design of certain lightweight systems.

It is thus an object to provide a refrigeration system with a new and improved condenser fan speed control.

It is a further object to eliminate from a refrigeration system using a high-pressure cutout means for rendering said cutout ineffective at startup by using a variable-speed condenser fan.

It is another object to construct a refrigeration system having a variable-speed condenser fan with lightweight materials and lower cost.

Another object of the invention is to provide a refrigeration system with a variable-speed condenser fan using a directly sensing pressure-resistive transducer.

A further object is to construct the above-mentioned transducer with a piezoresistive component.

This invention involves a refrigeration apparatus comprising a refrigerant compressor; an air-cooled refrigerant condenser; refrigerant throttling means; and a refrigerant evaporator respectively serially connected in a closed refrigerant circuit wherein that portion of said refrigerant circuit extending from said compressor through said condenser to said throttling means is the high-pressure side; a fan disposed to pass cooling air over said refrigerant condenser; an electric fan motor drivingly connected to said fan; an alternating-current power source for said motor; a gated semiconductor switch; a first electric circuit means serially connecting said altemating-current power source, said gated semiconductor switch and said electric fan motor; a fluid pressure-electrical resistive transducer; means connecting said transducer to said high-pressure side of said refrigeration system to respond to the refrigerant pressure therein; and second electric circuit means including said electrical resistive transducer, said alternating-current power source, and the gate of said gated semiconductor switch connected for controlling said gate in response to the pressure at said transducer.

Other objects and advantages will become apparent as this specification proceeds to describe the invention with reference to the drawings in which:

FIG. I is a diagrammatic view of a refrigeration system and control circuitry therefor incorporating the invention;

FIG. 2 is an exemplary pressure-time chart for the refrigeration system of FIG. 1 operated in accordance with the teachings of this invention showing high side pressure in p.s.i. and time in minutes; and

FIG. 3 is a pressuretime chart for a similar system using a condenser temperature-responsive variable resistance for varying condenser fan speed under similar conditions.

Referring to FIG. 1, there is shown a refrigeration system 10 for cooling a conditioned space 12. Refrigeration system 10 includes a reciprocating refrigerant compressor 14, an aircooled refrigerant condenser 16, a refrigerant throttling means such as temperature responsive expansion valve 18, and refrigerant evaporator 20 respectively serially connected in a closed refrigerant loop or circuit. Compressor 14 includes a suction valve 22 for receiving refrigerant gas from evaporator 20 and a discharge valve 24 for discharging compressed refrigerant gas to condenser 16. Compressor 14 also includes a piston 26 for compressing refrigerant in a compression cylinder 27 communicating with valves 22 and 24. Piston 26 is reciprocally driven by electric motor 28.

Condenser cooling fan 30 is arranged to pass cooling air over condenser 16 and is driven by electric motor 32. Compressor motor 28 and fan motor 32 are connected to an electric control circuit network 34 including an altemating-current power source 35, a thermostatic switch 36 responsive to the temperature of conditioned space 12, a high-pressure cutout switch 38 arranged to open in response to abnormally high pressure in the high side, i.e., that portion of the refrigerant circuit extending from the compressor 14 through condenser 16 to throttling means 18 and a symmetrical gated switch 40 such as General Electric switch SC45B/SC46B. The circuit network 34 also includes components for controlling the operation of switch 40 including capacitor 42, pressure-resistive transducer 44 and a bidirectional diode switch 46 such as General Electric switch Z1238.

Electric control network 34 presents a first electric circuit serially connecting altemating-current power source 35, thermostatic switch 36, high-pressure cutout switch 38, and compressor motor 28; and a second electric circuit serially connecting alternating-current power source 35, thermostatic switch 36, symmetrical gated switch 40 and condenser fan motor 32. A third circuit shunt switch 40 and includes transducer 44 and capacitor 42. The gate of switch 40 is connected to a point between transducer 44 and capacitor 42 via diode switch 46. Transducer 44 is comprised of a bellows actuator 48 connected internally to the high-pressure side of the refrigeration system, and a semiconductor piezoresistive element 50 which may be in the form of a crystal positioned to be compressed by expansion of bellows 48 supported in frame member 52. Element 50 presents a resistance in the shunt circuit which varies in response to the pressure exerted thereon by bellows 48 with refrigerant high side pressure.

OPERATION As the temperature in conditioned space 12 rises to a predetermined temperature, thermostatic switch 36 closes completing the aforesaid first circuit through normally closed cutout switch 38 whereby compressor motor 28 is energized and the piston 26 of compressor 14 is caused to reciprocate in the cylinder 27. On downward strokes of piston 26, refrigerant gas is drawn from evaporator 20 through suction valve 22 into cylinder 27. Upon the upward strokes of piston 26, the refrigerant gas is compressed and discharged by way of discharge valve 24 to condenser 16.

As the pressure builds up in condenser 16 to the desired predetermined minimum condensing pressure, bellows 48 expands and changes the stress on piezoresistive element 50 thereby reducing the resistance thereof. As the resistance of element 50 is reduced, a predetermined voltage at capacitor 42 is reached earlier in each half-cycle. Bidirectional diode switch 46 serves to conduct only when the predetermined voltage is exceeded thereby triggering switch 40 only during the period of each half-cycle succeeding the instant when the predetermined voltage is exceeded. The predetermined voltage is set by the electrical characteristics of the diode switch 46. Thus as predetermined voltage on capacitor 42 is reached earlier in each half-cycle because of the lowering of the resistance of element 50, switch 40 transmits more current to fan motor 32. The speed of fan 30 is thus controlled in direct relation to the pressure in the high-pressure side.

When evaporator 18 cools conditioned space 12 to a predetermined temperature, thermostatic switch 36 opens deenergizing fan motor 32 and compressor motor 28 thereby terminating operation of the refrigeration system. When the temperature of the conditioned space again rises above the predetermined temperature the refrigeration system again is cycled into operation during which period the fan motor speed is controlled by piezoresistive element 50.

As the refrigeration system of FIG. I is cycled between operating and nonoperating condition, the refrigerant pressure in the high-pressure side will vary. An example of such pressure variation is shown in the chart of FIG. 2. It should be noted that upon energization of compressor motor 27, the pressure rises quickly to a predetermined control pressure of about 200 p.s.i. This pressure is the minimum desired operating pressure and is set by the selection of pressure-resistive transducer 44, capacitor 42, and bidirectional diode switch 46. The pressure is maintained at this level by variation of the fan speed. Thus as the load on the condenser increases, the slight increase in pressure increases the fan speed. Similarly as the load on the condenser decreases, the slight decrease in pressure decreases the fan speed. If the condenser load increases beyond that required to operate the fan at full speed, the pressure will rise moderately above the minimum control pressure commensurate with the load. However, for purposes of simplicity the chart has not been complicated by such latter mentioned operating conditions. When the refrigeration system is cycled to the off condition the pressure drops rapidly but at a decreasing rate until the high and low sides of the system are equalized. It is important to note from FIG. 2 that there is no excessive transient pressure at the start of each cycle which could actuate the high-pressure cutout. An exemplary cutout pressure for the system may be about 380 p.s.i. although this may vary substantially from one system to another and also with the type of high-pressure protection used.

Now referring to FIG. 3, a similar chart is shown from the same system operated under similar conditions wherein the condenser fan is controlled indirectly via condenser temperature in accordance with the prior art. Note that upon each energization of the compressor, the pressure rises rapidly and far beyond the minimum control pressure. This pressure may far exceed those pressures resulting from fully loading the condenser. This excess pressure results from the delay in condenser fan staring due to the thermal lag at the temperature sensor. Since this transient pressure is dependent upon the thermal delay of the sensor, and if the delay be sufiicient, the upper pressure limits of the system may be exceeded. When the fan finally starts, the pressure falls substantially below the predetermined control pressure. The pressure continues to hunt above and below the control pressure with wide swings for approximately 5 minutes. This hunting action produces excessive fan speed variations which are objectionable from the noise standpoint. This increase in pressure may also exceed the cutout pressure causing the system to shut down.

If a high-pressure cutout is used in systems represented by FIG. 3, it may be necessary to provide means to render the safety cutout device ineffective for a short period immediately following starting of the compressor lest the system will shut down in response to the abnormally high pressure. In the alternative, the condenser fan may be arranged to start at full speed. However, in residential applications, such alternative system is unnecessarily noisy especially for nighttime operation. FUrthermore the FIG. 3 type system must be designed for higher pressure and the compressor must be designed for higher pressure and the compressor must be capable of pumping against a substantially higher head. All of these disadvantages are avoided by use of my invention which eliminates the excessive transient pressure on compressor starting.

Thus the instant invention provides a refrigeration system which may be lighter in construction, requires a smaller compressor motor, eliminates premature cutout of the compressor, and is generally safer and quieter to operate.

Having now described in detail the preferred embodiment of the invention, it is contemplated that many changes may be made without departing from the scope or spirit of the invention which is limited only by the claims.

Iclaim:

l. A refrigeration apparatus comprising a refrigerant compressor; an air-cooled refrigerant condenser; refrigerant throttling means; and a refrigerant evaporator respectively serially connected in a closed refrigerant circuit wherein that portion of said refrigerant circuit extending from said compressor through said condenser to said throttling means is the highpressure side; a fan disposed to pass cooling air over said refrigerant condenser; an electric fan motor drivingly connected to said fan; an alternating-current power source for said motor, a gated semiconductor switch; a first electric circuit means serially connecting said alternating-current power source, said gated semiconductor switch and said electric fan motor; a fluid pressure-electrical resistive transducer; means connecting said transducer to said high-pressure side of said refrigeration system to respond to the refrigerant .pressure therein; and second electric circuit means including said electrical resistive transducer, said alternating current power source, and the gate of said gated semiconductor switch connected for controlling said gate in response to the pressure at said transducer.

2. The apparatus as defined in claim I wherein said transducer includes a piezoresistive component.

3. The apparatus as defined by claim 2 wherein said piezoresistive component is a semiconductor.

4. The apparatus as defined by claim 3 wherein said semiconductor is a crystalline element.

5. The apparatus as defined by claim I wherein said gated semiconductor switch is a symmetrical gated switch and said second electric circuit means includes a directional diode switch in series with the gate of said symmetrical gated switch.

6. A refrigeration apparatus comprising: a refrigerant compressor; an air-cooled refrigeration condenser, refrigerant throttling means; and a refrigerant evaporator respectively serially connected in a cooled refrigerant circuit wherein that portion of said refrigerant circuit extending from said compressor through said condenser to said throttling means is the high-pressure side; a fan disposed to pass cooling air over said refrigerant condenser; a first electric motor drivingly connected to said fan; an alternating-current power source; a first electric circuit for said electric motor; a second electric circuit for said second electric motor; control means connected to said power source for cyclically energizing said first and second circuits in response to the load on said refrigeration apparatus; protector means for said pressure in said high-pressure side; means for operating said fan to maintain the refrigerant pressure in said high-pressure side below said abnonnally high refrigerant pressure immediately following energization of said first circuit and above a predetermined minimum pressure during the run cycle of said refrigeration apparatus; said last-mentioned means including a pressureelectrical transducer responsive to variations in refrigerant pressure of said high-pressure side, and a gated semiconductor switch in said second circuit connected for varying the current to said second motor and thus the speed of said fan in response to the electrical side of said pressure-electrical transducer.

7. In a refrigeration system, a condenser, means for circulating a refrigerant through said condenser, a fan for circulating air across said condenser to reduce the temperature of and thereby condense the refrigerant circulating therethrough, an electric motor for driving said fan, and control means including an element capable of directly sensing the head pressure in said refrigeration system and having an electrical output with a magnitude proportional to the sensed pressure, switch means connected in series with the fan motor and source of operating voltage, whereby said motor is connected across said source of operating voltage and thereby energized when said switch means is closed, and cyclically operating switchcontrolling circuit means for opening and closing said switch means and for varying the duration of the period for which said switch means is closed in each operating cycle of said circuit means as the magnitude of the output of the sensing element varies, whereby the speed of the fan motor is varied as the sensed pressure changes to prevent the pressure in the system from falling below a predetermined minimum.

8. In a refrigeration system, a condenser means for circulating a refrigerant through said condenser, a fan for circulating air across said condenser to reduce the temperature of and whereby condense the refrigerant circulating therethrough, an electric motor for driving said fan, and control means for varying the speed of said motor and thereby the fan speed as the head pressure changes in said system to prevent the pressure from falling below a predetermined minimum during operation of said system, said control means including an element capable of directly sensing the head pressure in said refrigeration system, switch means connected in series with the fan motor and a source of operating voltage, whereby said motor is connected across said source of operating voltage and thereby energized when said switch means is closed and switch-controlling circuit means for alternatively opening and closing said switch means and for varying the duration of the period for which said switch means is closed in each operating cycle thereof as the pressure sensed by said sensing element varies, whereby the speed of the fan motor is varied as the sensed pressure changes.

9. The combination of an electric fan motor and control means for regulating the speed of the fan motor as to vary said speed in accord with changes in the pressure of a specified body of fluid, said control means including switch means adapted to connect the fan motor in series with a source of operating voltage, whereby said fan motor is connected across said source of operating voltage and thereby energized when said switch means is closed and cyclically operating switchcontrolling circuit means including a sensing element directly responsive to pressure for opening and closing said switch means and for varyin the duration of the period for which said switch means 18 c osed m each operatmg cycle of said CH- cuit means as the pressure sensed by said sensing element varies, whereby the speed of the fan motor is varied as the sensed pressure changes.

10. In a refrigeration system, a condenser, means for circulating a refrigerant through said condenser, a fan for circulating air across said condenser to reduce the temperature of and thereby condense the refrigerant circulating therethrough, an electric motor for driving said fan, and control means including a pressure-responsive element capable of directly sensing the head pressure in said refrigeration system for varying the speed of said fan and the head pressure changes to prevent pressure from falling below a predetermined minimum during operation of said system, switch means connected between the fan motor and a source of operating voltage, whereby said motor is connected across said source of operating voltage and thereby energized when said switch means is closed, a source of control voltage, and cyclically operating circuit means connected between said source of control voltage and said switch means for opening and closing said switch means, said circuit means including means for varying the duration of the period for which said switch means is closed in each operating cycle of said circuit means as the control voltage applied to said circuit means is varied, said pressure-responsive element including a variable resistor connected between said control voltage source and said circuit means and means for varying the resistance of said resistor and therefore the control voltage applied to said circuit means as the sensed pressure changes, thereby varying the duration of the period for which the switch means is closed in each operating cycle of the circuit means and therefore the fan motor speed as the sensed pressure changes.

UNITED STATES PATENT @FFTCE widows CQECHON Patent No. 3,633,376 Dated Januarv 11, 1972 lrtventor(s) RobertG, Miner It is certified that error appears in the above-identified patent and that said Letters Patent are hereby'corrected as shown below:

column 4 line 72,. "directional" should read unidirectional line 74, "condenser, should read condenser; 6 Column 5, line 2, "cooled" should read closed line 7, before "fan", insert compressor; a second electric motor drivingly connected to said line 8, after said insert first line 12, cancel "pressure" and insert refrigeration apparatus responsive to an abnormally high pressure line 20, "side," should read side;

Signed and sealed this 14th day of November 1972,

(SEAL) Attest:

EDWARD M .FLETCHER,JR. v 1 ROBERT GOT'ISCHALK Attesting Officer Commissioner of Patents EJNHED STATES PATENT @FFEQE CEHNQA'EE @F QQEQTWN Patent No. 3,633,376 Dated Januarv 11,, 1972 Iriventor(s) Robert Go Miner It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 72,, "directional" should read unidirectional line 74, "condenser," should read condenser; a Column 5, line 2, "cooled" should read closed line 7 before "fan", insert compressor; a second electric motor drivingly connected to said line 8, after "said" insert first line 12, cancel "pressure and insert refrigeration apparatus responsive to an abnormally high pressure line 20, "side," should read side; 0'

Signed and sealed this 14th day of November 1972a (SEAL) Attest:

EDWARD M.FLETCHER,JR ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents FORM USCOMM-DC 60376-P69 U GOVYIYNMENT FRINTING OFFICE: 959 0-356334, 

1. A refrigeration apparatus comprising a refrigerant compressor; an air-cooled refrigerant condenser; refrigerant throttling means; and a refrigerant evaporator respectively serially connected in a closed refrigerant circuit wherein that portion of said refrigerant circuit extending from said compressor through said condenser to said throttling means is the high-pressure side; a fan disposed to pass cooling air over said refrigerant condenser; an electric fan motor drivingly connected to said fan; an alternating-current power source for said motor, a gated semiconductor switch; a first electric circuit means serially connecting said alternating-current power source, said gated semiconductor switch and said electric fan motor; a fluid pressure-electrical resistive transducer; means connecting said transducer to said high-pressure side of said refrigeration system to respond to the refrigerant pressure therein; and second electric circuit means including said electrical resistive transducer, said alternating current power source, and the gate of said gated semiconductor switch connected for controlling said gate in response to the pressure at said transducer.
 2. The apparatus as defined in claim 1 wherein said transducer includes a piezoresistive component.
 3. The apparatus as defined by claim 2 wherein said piezoresistive component is a semiconductor.
 4. The apparatus as defined by claim 3 wherein said semiconductor is a crystalline element.
 5. The apparatus as defined by claim 1 wherein said gated semiconductor switch is a symmetrical gated switch and said second electric circuit means includes a directional diode switch in series with the gate of said symmetrical gated switch.
 6. A refrigeration apparatus comprising: a refrigerant compressor; an air-cooled refrigeration condenser, refrigerant throttling means; and a refrigerant evaporator respectively serially connected in a cooled refrigerant circuit wherein that portion of said refrigerant circuit extending from said compressor through said condenser to said throttling means is the high-pressure side; a fan disposed to pass cooling air over said refrigerant condenser; a first electric motor drivingly connected to said fan; an alternating-current power source; a first electric circuit for said electric motor; a second electric circuit for said second electric motor; control means connected to said power source for cyclically energizing said first and second circuits in response to the load on said refrigeration apparatus; protector means for said pressure in said high-pressure side; means for operating said fan to maintain the refrigerant pressure in said high-pressure side below said abnormally high refrigerant pressure immediately following energization of said first circuit and above a predetermined minimum pressure during the run cycle of said refrigeration apparatus; said last-mentioned means including a pressure-electrical transducer responsive to variations in refrigerant pressure of said high-pressure side, and a gated semiconductor switch in said second circuit connected for varying the current to said second motor and thus the speed of said fan in response to the electrical side of said pressure-electrical transducer.
 7. In a refrigeration system, a condenser, means for circulating a refrigerant through said condenser, a fan for circulating air across said condenser to reduce the temperature of and thereby condense the refrigerant circulAting therethrough, an electric motor for driving said fan, and control means including an element capable of directly sensing the head pressure in said refrigeration system and having an electrical output with a magnitude proportional to the sensed pressure, switch means connected in series with the fan motor and source of operating voltage, whereby said motor is connected across said source of operating voltage and thereby energized when said switch means is closed, and cyclically operating switch-controlling circuit means for opening and closing said switch means and for varying the duration of the period for which said switch means is closed in each operating cycle of said circuit means as the magnitude of the output of the sensing element varies, whereby the speed of the fan motor is varied as the sensed pressure changes to prevent the pressure in the system from falling below a predetermined minimum.
 8. In a refrigeration system, a condenser means for circulating a refrigerant through said condenser, a fan for circulating air across said condenser to reduce the temperature of and whereby condense the refrigerant circulating therethrough, an electric motor for driving said fan, and control means for varying the speed of said motor and thereby the fan speed as the head pressure changes in said system to prevent the pressure from falling below a predetermined minimum during operation of said system, said control means including an element capable of directly sensing the head pressure in said refrigeration system, switch means connected in series with the fan motor and a source of operating voltage, whereby said motor is connected across said source of operating voltage and thereby energized when said switch means is closed, and switch-controlling circuit means for alternatively opening and closing said switch means and for varying the duration of the period for which said switch means is closed in each operating cycle thereof as the pressure sensed by said sensing element varies, whereby the speed of the fan motor is varied as the sensed pressure changes.
 9. The combination of an electric fan motor and control means for regulating the speed of the fan motor as to vary said speed in accord with changes in the pressure of a specified body of fluid, said control means including switch means adapted to connect the fan motor in series with a source of operating voltage, whereby said fan motor is connected across said source of operating voltage and thereby energized when said switch means is closed and cyclically operating switch-controlling circuit means including a sensing element directly responsive to pressure for opening and closing said switch means and for varying the duration of the period for which said switch means is closed in each operating cycle of said circuit means as the pressure sensed by said sensing element varies, whereby the speed of the fan motor is varied as the sensed pressure changes.
 10. In a refrigeration system, a condenser, means for circulating a refrigerant through said condenser, a fan for circulating air across said condenser to reduce the temperature of and thereby condense the refrigerant circulating therethrough, an electric motor for driving said fan, and control means including a pressure-responsive element capable of directly sensing the head pressure in said refrigeration system for varying the speed of said fan and the head pressure changes to prevent pressure from falling below a predetermined minimum during operation of said system, switch means connected between the fan motor and a source of operating voltage, whereby said motor is connected across said source of operating voltage and thereby energized when said switch means is closed, a source of control voltage, and cyclically operating circuit means connected between said source of control voltage and said switch means for opening and closing said switch means, said circuit means including means for varying the duration of the period for which said switch means is closed In each operating cycle of said circuit means as the control voltage applied to said circuit means is varied, said pressure-responsive element including a variable resistor connected between said control voltage source and said circuit means and means for varying the resistance of said resistor and therefore the control voltage applied to said circuit means as the sensed pressure changes, thereby varying the duration of the period for which the switch means is closed in each operating cycle of the circuit means and therefore the fan motor speed as the sensed pressure changes. 